Conventionally, there has been known an electric power steering system that assists steering operation by a driver.
An electric power steering system disclosed in Patent Literature 1 (JP 2015-93512 A) includes a rack housing, which houses a rack shaft and a ball screw mechanism disposed in the rack shaft, and a motor, which is fixed to the rack housing. A motor shaft protruding from a motor case is inserted into the rack housing. Inside the rack housing, a belt is wound between a pulley fixed to an end of the motor shaft and the ball screw mechanism. A torque of the motor is transferred to the rack shaft from the motor shaft through the pulley, the belt, and the ball screw mechanism. As a result, steering operation by a driver is assisted by the torque of the motor.
The electric power steering system includes a first annular packing and a second annular packing disposed at a connection point of the rack housing and the motor case. The first packing and the second packing prevent water (hereinafter “impure water”) containing foreign matter such as sand and mud from entering into an inside of the rack housing from an outside thereof.
However, there may be concern in the electric power steering system recited in Patent Literature 1 about entry of the impure water into the inside of the rack housing through worn boots that are disposed at both ends of the rack housing and worn out due to an impact by foreign matter during vehicle operation or due to aged deterioration. In this case, the impure water collected inside the rack housing may further enter into the motor case through a hole of the motor case into which the motor shaft is inserted. As a result, mechanical locking or electrical short due to rust generated inside the motor may occur.
In a case where the motor integrally has a controller that controls energization of the motor, there may be concern about occurrence of a variety of troubles such as stop of the electric power steering system.
In view of the above, it is an objective of the present disclosure to provide a driving unit that prohibits enter of the impure water containing water and foreign matter and to provide an electric power steering system using the driving unit.